# std::projected

From cppreference.com
< cpp‎ | iterator

Defined in header `<iterator>` |  |
| (1) |
template< std::indirectly_readable I, std::indirectly_regular_unary_invocable<I> Proj > struct projected { using value_type = std::remove_cvref_t<std::indirect_result_t<Proj&, I>>; std::indirect_result_t<Proj&, I> operator*() const; // not defined }; |  | (since C++20) (until C++26)
template< std::indirectly_readable I, std::indirectly_regular_unary_invocable<I> Proj > using projected = /*projected-impl*/<I, Proj>::/*__type*/; // see (3) |  | (since C++26)
template< std::weakly_incrementable I, class Proj > struct incrementable_traits<std::projected<I, Proj>> { using difference_type = std::iter_difference_t<I>; }; | (2) | (since C++20) (until C++26)
template< class I, class Proj > struct /*projected-impl*/ { struct /*__type*/ { using value_type = std::remove_cvref_t<std::indirect_result_t<Proj&, I>>; using difference_type = std::iter_difference_t<I>; // conditionally present std::indirect_result_t<Proj&, I> operator*() const; // not defined }; }; | (3) | (since C++26) (exposition only*)

1) Class(until C++26)Alias(since C++26) template `projected` combines an `indirectly_readable` type `I` and a callable object type `Proj` into a new `indirectly_readable` type whose reference type is the result of applying `Proj` to the std::iter_reference_t<I>.
2) This specialization of std::incrementable_traits makes std::projected<I, Proj> a `weakly_incrementable` type when `I` is also a `weakly_incrementable` type.
3) An indirect layer used for avoiding unexpected argument-dependent lookup. The member type `difference_type` exists only if `I` models `weakly_incrementable`.

`projected` is used only to constrain algorithms that accept callable objects and projections, and hence its operator*() is not defined.

### Template parameters

I | - | an indirectly readable type
Proj | - | projection applied to a dereferenced `I`

### Notes

The indirect layer prevents `I` and `Proj` to be associated classes of `projected`. When an associated class of `I` or `Proj` is an incomplete class type, the indirect layer avoids the unnecessary attempt to inspect the definition of that type that results in hard error.

### Example

Run this code

    #include <algorithm>
    #include <cassert>
    #include <functional>
    #include <iterator>

    template<class T>
    struct Holder
    {
        T t;
    };

    struct Incomplete;

    using P = Holder<Incomplete>*;

    static_assert(std::equality_comparable<P>); // OK
    static_assert(std::indirectly_comparable<P*, P*, std::equal_to<>>); // Error before C++26
    static_assert(std::sortable<P*>); // Error before C++26

    int main()
    {
        P a[10] = {}; // ten null pointers
        assert(std::count(a, a + 10, nullptr) == 10); // OK
        assert(std::ranges::count(a, a + 10, nullptr) == 10); // Error before C++26
    }

### See also

projected_value_t (C++26) | computes the value type of an `indirectly_readable` type by projection (alias template)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/iterator/projected&oldid=170804"